Staking machines have been used for some time to soften leather. The leather is softened by staking plates which vibrate into each other. The leather is transported between these staking plates while the plates are vibrating. One of the major problems associated with such machines is that the drives can become overloaded thereby leading to destruction of the machine. Overloads occur when very heavy, hard and/or uneven leathers are transported between the staking plates. Overloads can also occur when extraneous elements are passed between the staking plates.
In prior art staking machines, a shear pin is attached to the pressure heads of each staking plate in order to prevent overloading of the staking machine. The use of shear pins, however, slows down the softening process because each time a pin breaks the staking machine must be shut off so that the broken pin can be replaced. Using thicker and stronger shear pins will prevent the pins from breaking easily, but thicker and stronger shear pins cause the critical point of overload-power to be increased thereby subjecting the drive (crank shafts, staking plates, connection rods, beams, etc.) to possible damage or destruction.
There have been several attempts made to secure the pressure heads with hydraulic cylinders in order to eliminate the use of shear pins. Use of hydraulic cylinders, however, involves complicated constructions and mountings which result in very high costs. Therefore, the use of hydraulic cylinders is only economically feasible when the entire machine is working on a hydraulic base already. Otherwise, a complete hydraulic unit must be installed just to prevent overload.
It is therefore a principle object of the present invention to provide an overload-security system which is essentially trouble-free and which does not include complicated constructions and mountings.